In recent years, there has been development of organic semiconducting (OSC) materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), organic photodetectors (OPDs), organic photovoltaic (OPV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example of between 50 and 300 nm thickness.
One particular area of importance is organic photovoltaics (OPV). Conjugated polymers have found use in OPVs as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based photovoltaic devices are achieving efficiencies above 8%.
However, the polymers for use in OPV or OPD devices that have been disclosed in prior art still leave room for further improvements, like a lower bandgap, better processability especially from solution, higher OPV cell efficiency, and higher stability.
A commonly used strategy to control the energy levels and band gaps of polymers is to utilize an alternating copolymer consisting of electron-rich and electron-poor units within the polymer backbone. One of the recently introduced acceptor units is the bithiophene imide (dithieno[3,2-c;2′,3′-e]azepine-4,6-dione) as shown below.

Polymers containing this unit have been applied as n-type OFET materials exhibiting mobilities up to 0.04 cm2/Vs and good stabilities (see J. A. Letizia et al., J. Am. Chem. Soc. 2008, 130, 9679-9694; J. A. Letizia et al., Adv Fund. Mater. 2010, 20, 50-58; X. Guo et al., J. Am. Chem. Soc. 2011, 133, 1405-1418; X. Guo et al., J. Am. Chem. Soc. 2012, 134, 18427-18439).
Alternating donor-acceptor copolymers containing bithiophene imide as the acceptor and dithienosilole as the donor have been prepared and have shown in blends with PC71 BM an efficiency up to 5.5% in inverted OPV devices, similar to an analogous copolymer with TPD (thienopyrrolidinedione) unit as the acceptor (see X. Guo et al., J. Am. Chem. Soc. 2012, 134, 18427-18439; N. Zhou et al., Adv. Mater. 2012, 24, 2242-2248). However, compared to TPD, bithiophene imide copolymers are more ordered in the solid state, but have lower hole mobilities.
Copolymers containing the bithiophene imide unit as electron accepting unit are disclosed in WO2011/025454 A1. Random copolymers of bithiophene imide as the acceptor unit for use in transistors and organic solar cells are disclosed in WO2013/142845 A1. Compounds and polymers containing the bithiophene imide unit are also disclosed in WO2009/115413 A2 and WO2010/136401 A2.
However, the OPV and OTFT performance of polymers incorporating bithiophene imide units is limited, which is presumed to be inter alia due to the relatively small size of the conjugated system and, hence, suboptimal π-stacking ability and too high reorganization energy.
Thus there is still a need for organic semiconducting (OSC) polymers which are easy to synthesize, especially by methods suitable for mass production, show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, a good processibility, especially a high solubility in organic solvents, and high stability in air. Especially for use in OPV cells, there is a need for OSC materials having a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies, compared to the polymers from prior art.
It was an aim of the present invention to provide compounds for use as organic semiconducting materials that are easy to synthesize, especially by methods suitable for mass production, which show especially good processibility, high stability, good solubility in organic solvents, high charge carrier mobility, and a low bandgap. Another aim of the invention was to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that one or more of the above aims can be achieved by providing conjugated polymers comprising one or more novel units based on a central azepine-4,6-dione-like ring fused with 2-3 aromatic rings on each side, in particular copolymers comprising one or more of these units as electron acceptor units and further comprising one or more electron donor units.
It was found that extension of the bithiophene imide unit by fusing additional aromatic or heteroaromatic rings leads to system having greater propensity to form close π-π contacts in the film, as well as benefiting from reduced electron and hole reorganization energies. Moreover, this modification allows tuning the energetics of this unit, yielding building blocks of desired electron accepting or donating properties.
Surprisingly it was found that donor-acceptor copolymers, comprising the novel units as disclosed and claimed hereinafter as acceptor units, provide several advantages. For example, they have an increased solubility profile in common organic solvents (and especially non-chlorinated solvents) leading to better processability, and exhibit a good solid state organisation leading to efficient charge transport. The incorporation of electron donor units in addition to the novel acceptor units in the polymer backbone can lead to increased light absorption.